A conventional phase sensor detects the geometry of a sensor wheel. For this purpose, a signal is evaluated which represents a magnetic variable that is recorded by the sensor when a tooth is passed by the sensor. The sensor recognizes that the tooth has passed by, and “switches,” i.e., recognizes this passing by when a predefined portion of a signal amplitude is exceeded in the magnetic signal detected by the sensor. Since the signal amplitude, and therefore the switching level, changes as a function of temperature, the air gap, or aging, the switching threshold must be continually readjusted during operation. Present sensors use a preprogrammed switching threshold during switch-on which ensures over all operating states that the sensor recognizes the tooth and the gap during switch-on. This allows the so-called “true power-on” function, in which the sensor recognizes during switch-on whether it is situated in front of a tooth or a gap. The sensor subsequently learns or trains the optimal switching threshold for recognizing a tooth-gap pair as a function of the instantaneously present magnetic signal amplitude.
German Patent No. DE 36 38 622 C2 describes a phase sensor which builds up its own magnetic field via a magnet and registers changes in the magnetic field.